There are various kinds of bicycle hydraulic systems. A bicycle hydraulic system for a bicycle hydraulic brake or the like includes a hydraulic operating device, which may be a brake lever device, a hydraulic operated device, which may be a brake caliper, and a hydraulic hose, which connects the hydraulic operating device and the hydraulic operated device. The hydraulic operating device includes a master cylinder and a cylinder bore that accommodates a master piston. The master piston moves in the cylinder bore from an initial position to an actuation position when an operation member such as a hand lever is pivoted. The movement of the master piston discharges a fluid (here, oil) out of the master cylinder. The fluid drives a slave piston accommodated in the brake caliper through the hydraulic hose. Consequently, the brake caliper generates braking force.
The hydraulic operating device may include a reservoir tank (also referred to as fluid reservoir). When the master piston is located at the initial position, the reservoir tank is in fluidal communication with the cylinder bore of the master cylinder through a communication port. When the master piston is located at the actuation position, the fluidal communication of the reservoir tank and the communication port with the cylinder bore of the master cylinder is interrupted.